1. Field of the Invention
This invention relates to secondary cells or batteries and more particularly, to hydrogen storage electrodes using hydrogen storage alloys or their hydrides capable of reversibly storing or absorbing and releasing hydrogen. The hydrogen storage electrodes can be utilized in alkaline batteries which are unlikely to produce public nuisance and have high energy densities.
2. Description of the Prior Art
Currently, lead acid storage batteries and alkaline storage batteries have been widely used as secondary batteries. Among the alkaline storage batteries, nickel-cadmium batteries are most widely employed. However, the recent development of portable appliances demands a secondary battery or cell of a higher energy density substantially free of any ecological problems.
Attempts have been made to use zinc as a negative electrode in place of cadmium. However, the cell using the zinc electrode has not been in wide use because of the problem of life cycle. Now, attention has been drawn to alkaline storage batteries using a hydrogen storage alloy capable of reversibly storing and releasing hydrogen as a negative electrode and a nickel oxide as a positive electrode. This battery may be called a nickel-hydrogen secondary battery. The hydrogen storage battery has a substantially dischargeable energy density higher than existing cadmium batteries and is free of any formation of dendrite as with Zn. In addition, the hydrogen storage battery can be fabricated by known methods. Accordingly, the hydrogen storage battery has been expected as a secondary battery which has a high energy density, long life cycle and low public nuisance.
Known hydrogen storage alloys useful as the hydrogen storage electrode include, for example, Ti.sub.2 Ni, TiNi, LaNi.sub.5, MnNi.sub.5, and CaNi.sub.5 with or without other additive elements. These alloys are described, for example, in The Journal of Less-Common Metals 129 (1987), pp. 13-30 and 131 (1987), pp. 311-319. Moreover, U.S. Pat. Nos. 4,551,400 and 4,716,088 describe improved alloys of the formula, Ti.sub.2-x Zr.sub.x V.sub.4-y Ni.sub.y, in which 0&lt;x.ltoreq.1.5 and 0.6.ltoreq.y.ltoreq.3.5.
Although Ti.sub.2 Ni, TiNi and substituted alloys thereof have a relatively high dischargeable capacity in electrochemical charge and discharge cycles, a Ti-rich stable phase is formed after repetition of the charge and discharge cycles. This leads to a cycle life problem. On the other hand, LaNi.sub.5, MnNi.sub.5 and substituted alloys thereof are low in hydrogen storage capacity, so that the discharge capacity is relatively small. Moreover, this type of alloy has further disadvantages in that a variation in performance is great relative to the variation in temperature, the alloy is expensive, and the purities of starting metals are not necessarily satisfactory. Secondary batteries using CaNi.sub.5 and substituted alloys thereof exhibit a high discharge capacity at an initial stage of the charge and discharge cycle, but considerably lower in performance, similar to those using Ti.sub.2 N and TiNi, after repetition of the charge and discharge cycle.
The alloys of Ti.sub.2-x Zr.sub.x V.sub.4-x Ni.sub.y gradually convert into more stable hydrides and, thus, an effective alloy phase reduces in amounts with a problem of the cycle life.